1. Field of the Invention
The present invention relates to an exposure controller of a digital camera, more specifically, to improving the pre-exposure operation in a digital camera which is performed to calculate an optimum exposure time (shutter speed) at which the main exposure is performed.
2. Description of the Related Art
Conventional digital cameras are provided with a CCD image sensor (image pick-up device) on which the image is captured. Since the dynamic range of the CCD image sensor is generally narrow, the shutter speed (charge accumulating time/exposure time) has to be precisely determined to capture a clear image. In order to determine the shutter speed precisely, a method of performing a pre-exposure before the main exposure is known in the art. Namely, an exposure calculation is performed to determine an optimum shutter speed in accordance with a picture signal obtained from the CCD image sensor by the pre-exposure. Note that in the main exposure, the image captured on the CCD image sensor is stored in a recording medium such as a memory.
In the case where the shutter speed for the pre-exposure is relatively slow due to lack of quantity of light from an object to be photographed, the time lag between the pre-exposure and the main exposure becomes large. As a result, electric power consumption necessary to keep holding each of the quick-return mirror, the shutter, and the diaphragm of the camera at a predetermined position increases, especially in a single-lens reflex electronic camera, which is not preferable.